Catalyst components for the stereospecific polymerization of olefins are widely known in the art. Concerning the polymerization of propylene, the most spread out catalyst family belongs to the Ziegler-Natta category and in general terms it comprises a solid catalyst component, constituted by a magnesium dihalide on which are supported a titanium compound and an internal electron donor compound, used in combination with an Al-alkyl compound. Conventionally however, when a higher crystallinity of the polymer is required, also an external donor (for example an alkoxysilane) is needed in order to obtain higher isotacticity. One of the preferred classes of internal donors is constituted by the esters of phthalic acid, diisobutylphthalate being the most used. The phthalates are used as internal donors in combination with alkylalkoxysilanes as external donor. This catalyst system gives good performances in terms of activity, isotacticity and xylene insolubility.
One of the problems associated with the use of this catalyst system is that the phthalates have recently raised concerns due to the medical issues associated with their use and some compounds within this class have been classified as source of heavy health problems.
Consequently, research activities have been devoted to discover alternative classes of internal donors for use in the preparation of catalyst components for propylene polymerization.
Internal donors are described in U.S. Pat. No. 7,388,061 and WO2010/078494 which both relate to esters of aliphatic or aromatic diols. Both the references do not describe possible alternatives to these structures. Moreover, in view of the fact that their basic structure is that of a diol, it is difficult to produce multifunction derivatives with different functional groups (ester/ether, ester/carbamate etc) because the two oxygens of the diol basic structure have identical reactivity.
Surprisingly, the applicant has found that a particular class of donors based on mercapto derivatives is suited to generate a wide arrange of molecules with equal or different functional groups which when used as internal donors generate catalysts showing an excellent balance of activity and stereospecificity.